1. Field of the Invention
The present invention relates to a municipal solid waste fuel steam generator. Particularly, the present invention is directed to a municipal solid waste fuel steam generator having waterwall furnace platens.
2. Description of Related Art
Waste-to-energy or Energy-from-waste plants generate high-energy steam in boilers by combusting waste materials. Municipal solid waste fuel steam generators use solid municipal waste, such as refuse, as fuel to generate steam. The steam is commonly used to drive high-pressure steam turbines in order to generate electricity or provide steam to a steam user “host” or district heating system. When operated in an environmentally friendly way, solid waste fuel steam generators can solve two problems at the same time: they benefit the environment by reducing the demand on landfills for disposal of solid waste and they generate much needed power.
Many steam generators, including those adapted to burn fossil fuels, municipal solid waste, and other fuels, include a superheater downstream from the combustion zone. Steam within the superheater reaches a superheated state as the passing combustion gases release heat into the superheater. This superheated steam is typically used to power high-pressure turbines. The material demands on the superheater in these extreme temperatures are great. One way to increase the life of superheaters in coal powered steam generators is to include a bank of waterwall furnace platens upstream from the superheater. The waterwall furnace platens circulate relatively cool fluids, and thereby absorb some of the heat from the combustion gases before the hot gases reach the superheater. This has the effect of lowering the furnace gas exit temperature in front of the superheater, extending its useable life. This arrangement ultimately reduces corrosion rate and extends the useful life of superheaters significantly and can be operated in a way that does not substantially impact overall thermal efficiency.
In municipal solid waste fuel steam generators, however, the combustion gases at the superheater are much more corrosive than in steam generators using coal and other fuels. This leads to the need for frequent repair or replacement of the superheater, which results in downtime. The highly corrosive environment has prevented use of waterwall furnace platens in steam generators using municipal solid waste fuel. Instead, other approaches have been made to protect the superheater, such as placing costly high alloy tubes and/or shields in the path of the combustion gases or making the overall height of the furnace greater in order to reduce the furnace flue gas exit temperature. However, in order to reduce corrosion of superheaters in existing facilities, it is impractical and extremely difficult to increase the size of the furnace adequately due to space limitations. And shielding the superheater does not reduce the furnace flue gas exit temperatures in the vicinity of the superheater.
Such conventional methods and systems have generally been considered satisfactory for their intended purpose. However, there still remains a continued need in the art for a waterwall furnace platen suitable for protecting superheaters within municipal solid waste fuel steam generators. There also remains a need in the art for a waterwall furnace platen for use in municipal solid waste fuel steam generators that is durable and that is easy and cost effective to make and use. The present invention provides a solution for these problems.